1. Field of the Invention
Embodiments of the present invention generally relate to solar cells and methods for forming the same. More particularly, embodiments of the present invention relate to a gallium arsenide (GaAs) based solar cells.
2. Description of the Related Art
Solar cells convert solar radiation and other light into usable electrical energy. The energy conversion occurs as the result of the photovoltaic effect. Solar cells may be formed from crystalline material or from amorphous or microcrystalline materials. Generally, there are two major types of solar cells that are produced in large quantities today, which are crystalline silicon solar cells and thin film solar cells. Crystalline silicon solar cells typically use either monocrystalline substrates (i.e., single-crystal substrates of pure silicon) or a multicrystalline silicon substrates (i.e., poly-crystalline or polysilicon). Additional film layers are deposited onto the silicon substrates to improve light capture, form the electrical circuits, and protect the devices. Suitable substrates include glass, metal, and polymer substrates. It has been found that the properties of thin-film solar cells degrade over time upon exposure to light, which can cause the device stability to be less than desired. Typical solar cell properties that may degrade are the fill factor (FF), short circuit current, and open circuit voltage (Voc).
Thin film silicon solar cells have gained a significant market share due to low-cost, large-area deposition of the amorphous-microcrystalline silicon absorber layers. Thin-film solar cells use thin layers of materials deposited on suitable substrates to form one or more p-n junctions. Generally, different material layers perform different functions in the solar cells. Some material layers are configured to reflect and scatter light in the solar cells formed on the substrate so as to assist light retaining in the solar cell for a longer current generation. In some instances, some material layers may serve as a light absorber layer that may have high light-trapping effect. The light-trapping effect absorbs light in the absorber layer to generate high current. In general, the light absorber layer is configured to absorb photons with minimum recombination before they reach the solar cell electrodes and generate photocurrent. However, some absorber layers are often defective with high densities of recombination sites that have a significant detrimental effect on the efficiency of the solar devices.
Therefore, there is a need for improved thin film solar cells and methods and apparatus for forming the same with minimum recombination of electron-hope pairs to improve conversion efficiency of the solar cell.